Controlling stations&#39; access to a communications medium

ABSTRACT

Managing access to a time-slotted communications medium by masking interrogatory codes that are contained in signals demarcating time slots of the medium and comparing the masked interrogatory codes against masked response codes. Access to the medium is dependent on the outcome of these comparisons.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation application of U.S. application Ser.No. 13/416,341, filed Mar. 9, 2012, entitled “CONTROLLING STATIONS'ACCESS TO A COMMUNICATIONS MEDIUM,” the contents of which areincorporated herein by reference.

FIELD

The invention relates to communications systems in which transmitterscompete for access to a transmission medium.

BACKGROUND

The Institute of Electrical and Electronic Engineers (IEEE) 802.11standards address systems in which a number of stations (STAs) competeto conduct communications via the resources of an access point (AP). Inthis context, the IEEE 802.11 standards specify a medium access controlsub-layer (a “MAC”) that is a carrier sense multiple access withcollision avoidance (CSMA/CA) based medium access scheme; when thenumber of STAs contending for the medium is very large, there will be ahigh probability of collisions and the spectrum is wasted by the failedtransmissions. The network throughput and packet transfer delay aredegraded dramatically.

There are some techniques that can potentially solve this problem. Someof them are based on centrally-scheduled medium access; some techniquesadapt the back-off window to match the number of STAs, which thenreduces the probability of two STAs throwing the same random back-offvalue; other techniques are based on grouping STAs and only allowingSTAs in the same group to contend for the medium. However, for thesetechniques to work, the AP needs to know how many STAs are in thenetwork, and this information is not available until STAs finish theirinitial association.

When a large number of STAs is trying to associate with the APsimultaneously (with a short period of time of arrival difference, e.g.after a wide-area power cut), there is increased probability that theSTAs will collide with each other and the medium could be congested. Ifthe AP tunes the parameters, such as back-off window size or number ofgroups, assuming a large number of STAs, the network throughput can alsobe compromised due to waste of spectrum resource in waiting fornon-existent STAs contending for medium. Therefore, there is a need fora scheme to manage the contentions from a potentially large number ofSTAs and a need for a scheme can to manage contentions before, as wellas after, association.

SUMMARY

According to one aspect, an embodiment of the invention provides amethod of controlling access of a station to a communications mediumthat is divided into time slots, the method comprising: a controller ofthe medium transmitting demarcation signals at regular intervals, eachdemarcation signal demarcating a respective time slot and containing oneor more interrogatory codes; the station receiving a demarcation signalcorresponding to a particular time slot; the station masking each of theone or more interrogatory codes from the received demarcation signalwith a control mask to produce for each interrogatory code a respectivemasked interrogatory code; the station masking a response code held bythe station with the control mask to produce a masked response code; andthe station barring itself from attempting to access the communicationsmedium in the particular time slot if none of the one or more maskedinterrogatory codes matches the masked response code. The invention thuscan provide a way of controlling when a station will attempt to access acommunications medium and, by extension, a way of controlling whengroups of stations will attempt to access a communications network.

According to another aspect, an another embodiment of the inventionprovides a method of operating a station intended to access acommunications medium that is divided into time slots by demarcationsignals transmitted from a controller at regular intervals, wherein eachdemarcation signal demarcates a respective time slot and contains one ormore interrogatory codes and the method comprises: the station receivinga demarcation signal corresponding to a particular time slot; thestation masking each of the one or more interrogatory codes from thereceived demarcation signal with a control mask to produce for eachinterrogatory code a respective masked interrogatory code; the stationmasking a response code held by the station with the control mask toproduce a masked response code; and the station barring itself fromattempting to access the communications medium in the particular timeslot if none of the one or more masked interrogatory codes matches themasked response code. The invention thus can provide a way ofcontrolling when a station will attempt to access a communicationsmedium and, by extension, a way of controlling when groups of stationswill attempt to access a communications network.

A method according to either aspect may further comprise the stationjoining a network maintained by the controller and the controller inresponse transmitting to the station a control signal that contains theresponse code. The control mask may, for example, be provided in, and byobtained from, the control signal. In another variant, a first mask isprovided in the control signal, a second mask is provided in thedemarcation signal that corresponds to the particular time slot, thefirst and second masks are binary words of equal length containing bitsin a one of a one state and a zero state and the method furthercomprises selecting the one of the first and second masks that containsthe most bits in the zero state to be the control mask.

In a method according to either of the stated aspects, it may be thecase that each demarcation signal contains a mask. In some variants, themask contained in the demarcation signal that corresponds to theparticular time slot is used by the station as the control mask. Inother variants, the station derives the control mask from the mask thatis contained in the demarcation signal that corresponds to theparticular time slot. In the case where the mask contained in thedemarcation signal that corresponds to the particular time slot is abinary word containing bits in a one of a one state and a zero state,the deriving of the control mask can for example comprise the stationchanging from a zero state to a one state one or more bits in thatbinary word.

In some embodiments, the controller chooses from a group ofpredetermined codes the one or more interrogatory codes that arecontained in each demarcation signal.

According to a yet further aspect, an embodiment of the inventionprovides a station for accessing a communications medium that is dividedinto time slots by demarcation signals transmitted from a controller atregular intervals, wherein each demarcation signal demarcates arespective time slot and contains one or more interrogatory codes andthe station comprises: a receiver for receiving signals from thecontroller; storage means for storing a response code; and processingmeans. The processing means is arranged to: recover one or moreinterrogatory codes from a demarcation signal received at the receiverand corresponding to a particular time slot of the medium; mask each ofthe one or more interrogatory codes from the received demarcation signalwith a control mask to produce for each interrogatory code a respectivemasked interrogatory code; mask the response code from the memory withthe control mask to produce a masked response code; and bar the stationfrom attempting to access the communications medium in the particulartime slot if none of the one or more masked interrogatory codes matchesthe masked response code. The invention thus can provide a way ofcontrolling when a station will attempt to access a communicationsmedium and, by extension, a way of controlling when groups of stationswill attempt to access a communications network.

In certain embodiments, the processing means is further arranged torecover the response code from a control signal that received at thereceiver from the controller as a consequence of the station joining anetwork maintained by the controller. In some variants of suchembodiments, the processing means is further configured to recover thecontrol mask from the control signal. In other variants, a first mask isprovided in the control signal, a second mask is provided in thedemarcation signal that corresponds to the particular time slot, thefirst and second masks are binary words of equal length containing bitsin a one of a one state and a zero state and the processing means isfurther arranged to select the one of the first and second masks thatcontains the most bits in the zero state to be the control mask.

Each demarcation signal may contain a mask and, in some variants, theprocessing means may be further arranged to use the mask contained inthe demarcation signal that corresponds to the particular time slot asthe control mask. In other variants, however, the processing means maybe further configured to derive the control mask from the mask that iscontained in the demarcation signal that corresponds to the particulartime slot. In the case where the mask contained in the demarcationsignal that corresponds to the particular time slot is a binary wordcontaining bits in a one of a one state and a zero state, the processingmeans can be configured to derive the control mask by changing from azero state to a one state one or more bits in the binary word.

The control mask, the response code and the one or more interrogatorycodes can be binary words.

The response code may be created by hashing a MAC address of thestation. The response code may be a random number.

According to yet another aspect, there are no control masks and anembodiment of the invention provides a method of controlling access of astation to a communications medium that is divided into time slots, themethod comprising: a controller of the medium transmitting demarcationsignals at regular intervals, each demarcation signal demarcating arespective time slot and containing one or more interrogatory codes; thestation receiving a demarcation signal corresponding to a particulartime slot; and the station barring itself from attempting to access thecommunications medium in the particular time slot if none of the one ormore interrogatory codes matches the response code. The invention thuscan provide a way of controlling when a station will attempt to access acommunications medium and, by extension, a way of controlling whengroups of stations will attempt to access a communications network.

According to a still further aspect, there are no control masks and anembodiment of the invention provides a method of operating a stationintended to access a communications medium that is divided into timeslots by demarcation signals transmitted from a controller at regularintervals, wherein each demarcation signal demarcates a respective timeslot and contains one or more interrogatory codes and the methodcomprises: the station receiving a demarcation signal corresponding to aparticular time slot; and the station barring itself from attempting toaccess the communications medium in the particular time slot if none ofthe one or more interrogatory codes matches the response code. Theinvention thus can provide a way of controlling when a station willattempt to access a communications medium and, by extension, a way ofcontrolling when groups of stations will attempt to access acommunications network.

According to a still further aspect, an embodiment of the inventionprovides a station for accessing a communications medium that is dividedinto time slots by demarcation signals transmitted from a controller atregular intervals, wherein each demarcation signal demarcates arespective time slot and contains one or more interrogatory codes andthe station comprises: a receiver for receiving signals from thecontroller; storage means for storing a response code; and processingmeans. The processing means is arranged to: recover one or moreinterrogatory codes from a demarcation signal received at the receiverand corresponding to a particular time slot of the medium; and bar thestation from attempting to access the communications medium in theparticular time slot if none of the one or more interrogatory codesmatches the response code. The invention thus can provide a way ofcontrolling when a station will attempt to access a communicationsmedium and, by extension, a way of controlling when groups of stationswill attempt to access a communications network.

By way of example only, certain embodiments will now be described byreference to the accompanying drawings, in which:

FIG. 1 is a block diagram schematically illustrating a network;

FIG. 2 is a timing diagram explaining some activity in the network ofFIG. 1;

FIG. 3 is a block diagram schematically illustrating the access point ofthe network shown in FIG. 1;

FIG. 4 is a block diagram schematically illustrating a station of thenetwork shown in FIG. 1; and

FIG. 5 is a plot illustrating the relative performance of differencetypes of medium access control schemes.

DETAILED DESCRIPTION

FIG. 1 illustrates a network 10 in which five STAs 12, 14, 16, 18 and 20compete using a CSMA/CA MAC scheme in order to conduct communicationsusing the resources provided by the AP 22. It will be readily apparentthat there could be more or fewer than five STAs in the network. The AP22 could, for example, be a wireless router providing a WiFi network andthe STAs 12-20 could, for example, be laptops, smart phones, printers ortablet computers that wish to conduct communications via the AP's WiFinetwork.

The AP 22 provides a communications medium through which the STAs 12-20can attempt to conduct communications. The communications mediumprovided by the AP 22 is divided into time slots. The beginning of eachslot is demarcated by a beacon frame. Each slot is given a group number,denoted v, that is eight bits long, and a mask, denoted m, that is alsoeight bits long. A slot's group number and its mask are communicated inits beacon frame. The AP 22 may arbitrarily change one or both of m andv for every slot to adjust the number of STAs that are allowed tocontend for access to the medium in each slot. The adjustment of m and vcan be based on observations of the medium made by the AP 22, onparameters such as extended interframe space (EIFS), packet error rate(PER) and clear channel assessment (CCA).

In order to conduct communications using the resources of the AP 22, theSTAs 12-20 need to “associate” with the AP 22. That is to say, the STAs12-20 must identify themselves to, and submit to the control of, the AP22. After the STAs 12-20 are associated with the AP 22, then the STAs12-20 can conduct communications using the resources of the AP 22 asrepresented by the communications medium. The STAs 12-20 will contendfor the use of the communications medium at various times, and it ispossible (and indeed expected) that there will be occasions whereseveral of the STAs 12-20 attempt to use the medium simultaneously. ASTA, is said to experience a “collision” if it attempts to access themedium and discovers that the medium is already in use by another one ofthe stations, say STA. A STA that experiences a collision, will “backoff”, that is to say, it will attempt to access the medium at some latertime instead.

The process by which a STA 12-20 contends for access to thecommunications medium provided by the AP 22 will now be discussed fromthe standpoint of STA 12. It is to be understood that the other STAs14-20 behave in the same way.

Before association with the AP 22, STA 12 generates an 8-bit randomnumber, denoted rv. As will be discussed later, the STA 12 does notchange this value until after successful association with the AP 22. TheSTA 12 only contends for the medium if (where ‘and’ denotes ‘bitwiseand’):

rv and m=v and m  condition {circle around (1)}

Once the STA 12 becomes associated with the AP 22, the AP 22 specifies anew rv value, denoted rv′, for the STA 12 to use after association. Thevalue rv′ does not change unless the AP 22 updates it using a new actionframe sent to the STA 12. (action frames are defined within the IEEE802.11 standards.) After association with the AP 22, the STA 12 does notcontend for the medium during a given slot unless it has received abeacon frame in that slot and (where ‘and’ denotes ‘bitwise and’):

rv′ and m=v and m  condition {circle around (2)}

The STA 12, is allowed, both before and after association, to makeautonomous adjustments to the mask so that the STA 12 can reduce itscontention to the medium and therefore reduce its overall collisionprobability without the intervention of the AP 22. This seeminglyself-sacrificing behaviour by STA 12 can improve the overallcommunications throughput of the network 10 as well as the individualaccess delay, in a statistically-averaged sense, that is experienced bythe STA 12. The autonomous adjustment that the STA 12 is permitted tomake is the changing of zero bits in the mask to ones, in which case themask m is modified and becomes a mask denoted m′. If the STA 12 has madesuch a change, then in a given slot, and prior to association of the STA12 with the AP22, the STA 12 will only contend for the medium if (where‘and’ denotes ‘bitwise and’):

rv and m′=v and m′  condition {circle around (3)}

Of course, if the STA 12 has become associated with the AP 22 and the APhas modified rv to rv′, then condition {circle around (3)} becomes:

rv′ and m′=v and m′  condition {circle around (4)}

A simple way for the STA 12 to modify its mask is to identify the zerobit nearest its least significant bit (LSB) end and then set that bit toone. For example, if one assumes that m=00010101 and that the LSB is atthe right-hand end, then m′=00010111. Other schemes are of coursepossible for choosing which additional bit to set to one. For example,the bit to alter could be found by searching from the most significantbit (MSB) end rather than the LSB end.

It is also possible for the network 10 to be configured to allow STA 12to change multiple zero bits into ones at the same time. It is alsopossible for the network 10 to be configured to allow the STA 12 to takea modified mask m′ created for use with one beacon frame and make afurther adjustment to the mask, by setting one or more zero bits to one,when using that mask with a subsequent beacon frame.

From one perspective, a beacon frame can be thought of as a demarcationsignal since it demarcates the time slots of the communications medium.Similarly, the group number sent in a beacon frame can be thought of asan interrogatory code since it is used by the STA 12 to query whetherthe STA can contend for access to the medium, and the rv value or, asthe case may be, the rv′ value can be thought of as a response codesince it governs the STA's response to the group number as provided bythe evaluation of the relevant one of conditions {circle around (1)} to{circle around (4)}. Moreover, the mask that is used in the evaluationof the relevant condition (m, m′ or, as will be discussed later, m″) canbe thought of as a control mask since it controls the comparison of theinterrogatory code with the group code. Assuming that the control mask,the interrogatory code and the response code are all equal in length,the more bits that are set to one in the control mask, less likely itbecomes that the STA 12 will be permitted to contend for access to thecommunications medium in a given time slot.

That concludes the explanation of the mechanism by which the STA 12contends for access to the communications medium provided by the AP 20.The other STAs 14-20 use the same way of contending for access to thecommunications medium.

FIG. 2 provides an example of a scenario where the STA 12-20 compete foraccess to the medium. FIG. 2 shows four consecutive slots in thecommunications medium. The slots are demarcated by their respectivebeacon frames 24, 26, 28 and 30, which are broadcast by the AP 22.

FIG. 2 illustrates the network 10 in action. FIG. 2 shows as anexemplary scenario a snapshot of four consecutive time slots 24, 26, 28and 30 in the medium, during which four of the five STAs 12-20 happen tocompete for access to the medium. Each of the time slots 24-30 commenceswith a respective beacon frame 32, 34, 36 and 38. The activity withinthese time slots will now be discussed.

Time Slot 24

During the course of this time slot, it happens that just STAs 12 and 16decide to contend for access to the medium. In this time slot, beaconframe 32 sets v=00000001 and m=00000001.

STA 12 has rv=00000011 and uses m as assigned by the beacon frame 32. Inthis instance:

rv and m=v and m

so the STA 12 is allowed to contend for access to the medium in thistime slot, and this contention is indicated by block 40.

STA 16 has rv=00000101 and, in view of the fact that it experienced acollision the last time that it attempted to access the medium, STA 16elects to use in this time slot a modified mask m′=00000011 in place ofm=00000001. In this instance:

rv and m′=v and m′

so the STA 16 is allowed to contend for access to the medium in thistime slot, and this contention is indicated by block 42.

Time Slot 26

During the course of this time slot, it happens that just STAs 12, 16and 18 decide to contend for access to the medium. In this time slot,beacon frame 34 sets v=00000011 and m=00000011.

STA 16 has rv=00000101 and uses m as assigned by the beacon frame 34. Inthis instance:

rv and m=v and m

so the STA 16 is not allowed to contend for access to the medium in thistime slot, and this prohibition is indicated by struck-through block 46.

STA 18 has rv=00000111 and uses m as assigned by the beacon frame 34. Inthis instance:

rv and m=v and m

so the STA 18 is allowed to allowed to contend for access to the mediumin this time slot, and this contention is indicated by block 48.

STA 12 has rv=00000011, it will be recalled, and in this time slotelects to use m as assigned by the beacon frame 34. In this instance:

rv and m=v and m

so the STA 12 is allowed to allowed to contend for access to the mediumin this time slot, and this contention is indicated by block 50.

Time Slot 28

During the course of this time slot, it happens that just STAs 12 and 14decide to contend for access to the medium. In this time slot, beaconframe 36 sets v=00000010 and m=00000010.

It so happens that STA 12 experienced a collision (with some otheractivity that is not shown in the Figure) when contending for access tothe medium in time slot 26, and therefore elects to use in this timeslot a modified mask m′=00000011 in place of m=00000010. STA 12 hasrv=00000011, it will be recalled, so, in this instance:

rv and m′≠v and m′

so the STA 12 is not allowed to contend for access to the medium in thistime slot, and this prohibition is indicated by struck-through block 52.

STA 14 has rv=00000110 and uses m as assigned by the beacon frame 36. Inthis instance:

rv and m=v and m

so the STA 14 is allowed to allowed to contend for access to the mediumin this time slot, and this contention is indicated by block 54.

Time Slot 30

During the course of this time slot, it happens that only STA 18 decidesto contend for access to the medium. In this time slot, beacon frame 38sets v=00000010 and m=00000010.

STA 18 has rv=00000111, it will be recalled, and elects to use m asassigned by the beacon frame 32. In this instance:

rv and m=v and m

so the STA 18 is allowed to contend for access to the medium in thistime slot, and this contention is indicated by block 56.

That concludes the description of the activity in the four time slots24-30. In the scenario shown in FIG. 2, the STAs 12-18 are notassociated with the AP 22; if they were so associated, then the relevantoccurrences of legend rv in that Figure would be replaced with rv′.

The operation of the network 10 as described above with reference toFIGS. 1 and 2 is useful in that it provides a way of controlling thetimes at which groups of STAs contend for access to the medium prior toassociation.

The operation of the network 10 as described above with reference toFIGS. 1 and 2 is useful in that it provides a way of controlling thetimes at which STAs contend for access to the medium after association.

The method of operating network 10 as described above with reference toFIGS. 1 and 2 is useful in that it can cope with large numbers of STAscontending for access to the medium.

FIG. 3 shows the AP 22. In FIG. 3, only the features of the AP 22 thatare most useful for describing the invention are shown. The skilledperson will readily understand that in practice the AP 22 will includemany more elements besides those shown in FIG. 3. As shown in FIG. 3,the AP 22 comprises a transmitter 58, a receiver 60, a processor 62 anda memory 64. The transmitter 58 receives communications (such asrequests from the STAs 12-20 for access to the communications mediumprovided the AP 22) from the network 10 and these are processed byprocessor 62 with the aid of instructions and data that are retrievedfrom the memory 64. The processor 62 can also store data andinstructions in the memory 64. The processor 62 can transmitcommunications (such as beacon frames containing m and v values for theSTAs 12-20 to use, or such as action frames containing rv′ and/or m″values for a STA to use) into the network 10 via the transmitter 58.

FIG. 4 shows the STA 12. In FIG. 4, only the features of the STA 12 thatare most useful for describing the invention are shown. The skilledperson will readily understand that in practice the STA 12 will includemany more elements besides those shown in FIG. 4 and that STAs 14-20 canhave the same design as STA 12. As shown in FIG. 4, the STA 12 comprisesa transmitter 66, a receiver 68, a processor 70 and a memory 72. Thetransmitter 66 receives communications (such as beacon frames containingm and v values for the STA 12 to use, or such as action framescontaining rv′ and/or m″ values for the STA 12) from the network 10 andthese are processed by processor 70 with the aid of instructions anddata that are retrieved from the memory 72. The processor 70 can alsostore data and instructions in the memory 72. The processor 70 cantransmit communications (such as requests to the AP 22 for access to thecommunications medium provided the AP 22) into the network 10 via thetransmitter 72.

FIG. 5 shows the results of a Monte-Carlo simulation that was run toevaluate the benefit of allowing STAs to contend for medium access ingroups, as in the scheme outlined with respect to FIGS. 1 and 2. In thesimulation, a given number of STAs simultaneously contended for themedium to transmit one packet; once its packet was deliveredsuccessfully, a STA kept silent. In the simulation, three MAC schemeswere evaluated:

-   -   data plot 40 shows the results for a basic distributed        coordination function (DCF) with an initial back-off window size        of 32.    -   data plot 42 shows the results for a basic DCF with an initial        back-off window size of 128.    -   data plot 44 shows the results for a CSMA/CA scheme in which:        -   the STAs are divided into twenty equal sized groups.        -   the communications medium is divided into time slots.        -   only one of the groups is allowed to contend for access to            the medium in each time slot, with the time slots repeatedly            cycling through the twenty groups such that in the n^(th)            time slot, the j^(th) group is allowed to contend, where j=n            modulo p and n is an integer greater than or equal to one            and p is the number of groups (twenty).        -   the STAs each have a fixed back-off window size of 32.

FIG. 53 shows that the number of collisions is reduced by grouping STAsbefore initial association (plot 44) indicating that the STA contentionscheme described by reference to FIGS. 1 and 2 can reduce the number ofcontentions significantly.

Various modifications can be made to the MAC scheme described byreference to FIGS. 1 and 2, and some of these will now be discussed.

In one embodiment, rv is not a random number generated by the particularSTA but instead is an eight bit hash of the STA's MAC address. In otherembodiments, different pseudorandom values can be used for rv.

In one embodiment, the mask m can be given a simpler form, inasmuch asit can instead be specified as a number of bits, commencing from the LSBend or the MSB end of m, that are to be the only non-zero bits in m. Forexample, a beacon frame could signal the number “three”, indicatingthat, for STAs that happen to contend in that beacon frame's time slot,m is an eight bit number with only the three LSBs set to one, i.e.,m=00000111. In this variant, the previously stated rules for adapting minto m′ can be used.

In one embodiment, the beacon frame includes a group of numbers v, andthe STAs check the masked rv or, as the case may be, masked rv′ againstthe list to decide whether it will contend in the beacon frame. A STAwill be able to contend if it finds a number in the list for which issatisfied the one of conditions {circle around (1)} to {circle around(4)} that it is using.

In one embodiment, once a STA is associated with the AP 22, the AP 22allocates that STA not only an rv′ value but also a mask m″ that the STAcan opt to use in place of m in the evaluation of condition {circlearound (2)}. In this embodiment, a STA will opt to use m″ in place of mfor evaluating condition {circle around (2)} unless m has fewer bits setto one than m″. A STA's rv′ and m″ values do not change unless the AP 22updates them using a new action frame sent to that STA. That is to say,a STA can modify m″ by changing bits from zero to one. In a variant ofthis embodiment, however, a STA can modify m″ by changing one or morebits from zero to one (which would reduce the probability that a STAcontends in a given time slot), but not by changing one or more bitsfrom one to zero (which would increase the probability that a STAcontends in a given time slot).

In another variant, the control mask used by a STA is set to all ones,e.g. 11111111 in the case where the control mask is an eight bit word.This “all ones” state could be used in one or more STAs and could beused for a period or indefinitely. In relation to the latter option, ina further and related variant, there is no control mask such thatconditions {circle around (1)} and {circle around (3)} both become rv=vand conditions {circle around (2)} and {circle around (4)} both becomerv′=v.

1. A method for controlling access of a station to a communicationsmedium that is divided into time slots, the method comprising:transmitting, by a controller of the medium, demarcation signals atregular intervals, each demarcation signal demarcating a respective timeslot and containing one or more interrogatory codes; receiving, by thestation, a demarcation signal corresponding to a particular time slot;masking, by the station, each of the one or more interrogatory codesfrom the received demarcation signal with a control mask to produce foreach interrogatory code a respective masked interrogatory code; andmasking, by the station, a response code held by the station andreceived from the controller with the control mask to produce a maskedresponse code; wherein the station bars itself from attempting to accessthe communications medium in the particular time slot if none of the oneor more masked interrogatory codes matches the masked response code. 2.A method according to claim 1, further including creating the responsecode by hashing a MAC address of the station.
 3. A method according toclaim 1, wherein the response code is a random number.
 4. A methodaccording to claim 1, further comprising the station joining a networkmaintained by the controller and the controller in response transmittingto the station a control signal that contains the response code.
 5. Amethod according to claim 4, further including obtaining the controlmask from the control signal.
 6. A method according to claim 4, whereina first mask is provided in the control signal, a second mask isprovided in the demarcation signal that corresponds to the particulartime slot, the first and second masks are binary words of equal lengthcontaining bits in a one of a one state and a zero state and the methodfurther comprises selecting the one of the first and second masks thatcontains the most bits in the zero state to be the control mask.
 7. Amethod according to claim 1, wherein the control mask, the response codeand the one or more interrogatory codes are binary words.
 8. A methodaccording to claim 1, wherein each demarcation signal contains a maskand the mask contained in the demarcation signal that corresponds to theparticular time slot is used by the station as the control mask.
 9. Amethod according to claim 1, wherein each demarcation signal contains amask and the method further comprises the station deriving the controlmask from the mask that is contained in the demarcation signal thatcorresponds to the particular time slot.
 10. A method according to claim9, wherein the mask contained in the demarcation signal that correspondsto the particular time slot is a binary word containing bits in a one ofa one state and a zero state and the step of the station deriving thecontrol mask comprises the station changing from a zero state to a onestate one or more bits in the binary word.
 11. A method according toclaim 1, further comprising the controller choosing from a group ofpredetermined codes the one or more interrogatory codes that arecontained in each demarcation signal.
 12. A method of operating astation intended to access a communications medium that is divided intotime slots by demarcation signals transmitted from a controller atregular intervals, wherein each demarcation signal demarcates arespective time slot and contains one or more interrogatory codes andthe method comprises: receiving, by the station, a demarcation signalcorresponding to a particular time slot; masking, by the station, eachof the one or more interrogatory codes from the received demarcationsignal with a control mask to produce for each interrogatory code arespective masked interrogatory code; and masking, by the station, aresponse code held by the station and received from the controller withthe control mask to produce a masked response code; wherein the stationbars itself from attempting to access the communications medium in theparticular time slot if none of the one or more masked interrogatorycodes matches the masked response code.
 13. A method according to claim12, further including creating the response code by hashing a MACaddress of the station.
 14. A method according to claim 12, wherein theresponse code is a random number.
 15. A method according to claim 12,further comprising the station joining a network maintained by thecontroller and consequently receiving from the controller a controlsignal that contains the response code.
 16. A method according to claim15, further comprising obtaining the control mask from the controlsignal.
 17. A method according to claim 15, wherein a first mask isprovided in the control signal, a second mask is provided in thedemarcation signal that corresponds to the particular time slot, thefirst and second masks are binary words of equal length containing bitsin a one of a one state and a zero state and the method furthercomprises the station selecting the one of the first and second masksthat contains the most bits in the zero state to be the control mask.18. A method according to claim 12, wherein the control mask, theresponse code and the one or more interrogatory codes are binary words.19. A method according to claim 12, wherein each demarcation signalcontains a mask and the mask contained in the demarcation signal thatcorresponds to the particular time slot is used by the station as thecontrol mask.
 20. A method according to claim 12, wherein eachdemarcation signal contains a mask and the method further comprises thestation deriving the control mask from the mask that is contained in thedemarcation signal that corresponds to the particular time slot.
 21. Amethod according to claim 20, wherein the mask contained in thedemarcation signal that corresponds to the particular time slot is abinary word containing bits in a one of a one state and a zero state andthe step of the station deriving the control mask comprises the stationchanging from a zero state to a one state one or more bits in the binaryword.
 22. A station for accessing a communications medium that isdivided into time slots by demarcation signals transmitted from acontroller at regular intervals, wherein each demarcation signaldemarcates a respective time slot and contains one or more interrogatorycodes and the station comprises: a receiver configured to receivesignals from the controller; a memory containing a response codereceived from the controller; and a processor configured to: a. recoverone or more interrogatory codes from a demarcation signal received atthe receiver and corresponding to a particular time slot of the medium;b. mask each of the one or more interrogatory codes from the receiveddemarcation signal with a control mask to produce for each interrogatorycode a respective masked interrogatory code; c. mask the response codefrom the memory with the control mask to produce a masked response code;and d. bar the station from attempting to access the communicationsmedium in the particular time slot if none of the one or more maskedinterrogatory codes matches the masked response code.
 23. A stationcorresponding to claim 22, wherein the response code is a hash of a MACaddress of the station.
 24. A station according to claim 22, wherein theresponse code is a random number.
 25. A station according to claim 22,wherein the processor is further configured to recover the response codefrom a control signal that received at the receiver from the controlleras a consequence of the station joining a network maintained by thecontroller.
 26. A station according to claim 25, wherein the processoris further configured to recover the control mask from the controlsignal.
 27. A station according to claim 25, wherein a first mask isprovided in the control signal, a second mask is provided in thedemarcation signal that corresponds to the particular time slot, thefirst and second masks are binary words of equal length containing bitsin a one of a one state and a zero state and the processor is furtherconfigured to select the one of the first and second masks that containsthe most bits in the zero state to be the control mask.
 28. A stationaccording to claim 22, wherein the control mask, the response code andthe one or more interrogatory codes are binary words.
 29. A stationaccording to claim 22, wherein each demarcation signal contains a maskand the processor is further configured to use the mask contained in thedemarcation signal that corresponds to the particular time slot as thecontrol mask.
 30. A station according to claim 22, wherein eachdemarcation signal contains a mask and the processor is furtherconfigured to derive the control mask from the mask that is contained inthe demarcation signal that corresponds to the particular time slot. 31.A station according to claim 30, wherein the mask contained in thedemarcation signal that corresponds to the particular time slot is abinary word containing bits in a one of a one state and a zero state andthe processor is configured to derive the control mask by changing froma zero state to a one state one or more bits in the binary word.